In a study of 130 cases of unilateral retinal vein occlusion uncomplicated by rubeosis, we found that more than 80% of the patients had a lower intraocular pressure (IOP) in the eye with the occlusion than in the fellow normal eye. The reduction of IOP was greater with central than with branch vein occlusion, greater with hemorrhagic than with venous stasis retinopathy, and greater in patients who had high pressures in their fellow eyes. The pressure reductions persisted during follow-up periods of up to two years. How retinal vein occlusion lowers IOP is obscure and may involve more than one mechanism. Outflow facility was increased (compared to the fellow eye) in hemorrhagic retinopathy and in branch vein occlusion, both of which are association with retinal ischemia, but not in venous stasis retinopathy. Calculated rates of aqueous formation were low in central vein occlusion but not in branch vein occlusion.